1. Field of the Invention
The present invention relates to an active energy ray-curable inkjet ink composition.
2. Description of the Related Art
Conventionally, active energy ray-curable inkjet inks, which can be dried more quickly than solvent-type inkjet inks, have been often used in signage printers for high speed printing. Such active energy ray-curable inkjet inks also have excellent adhesion to base materials and thus have been used in flat bed printers for printing on various base materials. Further, inks have been developed to meet applications.
These printers scan a printing head to meet an increase in size, in film thickness, or in high density. In recent years, as printing head technology has advanced, technology that enables a printing head to eject micro-droplets at high frequency has been established. With the merit of digitization, the achievement of this technology has increased the chance of replacing conventional printing methods with inkjet printing, which used to be inferior in productivity or image quality.
Moreover, a 1-pass type printer has been proposed in which inks are ejected from at least two or more of inkjet heads and then are cured at the same time. Accordingly, it is possible to realize low cost of equipment and high productivity and thus the replacing of conventional printing methods is further accelerated.
However, in order to realize replacing conventional printing methods with inkjet printing methods, it is necessary to develop an ink having high-frequency suitability better than that of conventional one and having low viscosity/high sensitivity, and stable ejection characteristics over a long time. High-frequency suitability, high sensitivity, and stable ejection characteristics for a long period of time contribute to an improvement in productivity, and thus are features required of the ink for achieving productivity as high as conventional printing. Lower ink viscosity contributes to an improvement in the accuracy of droplet landing, and thus is a feature required of the ink for achieving high-definition image quality. In addition, the substitution of inkjet printing for conventional offset printing also requires an improvement in color reproducibility. However, it has been very difficult, particularly for an active energy ray-curable inkjet ink, to satisfy all required properties including image color reproducibility, curing properties, discharge stability, and the like.
In a case where the amount of the application of ink composition droplets is increased in order to achieve wide color reproducibility, cracking of the cured film may occur to disturb the image or to degrade fixing properties. In a case where an ink composition is prepared with a higher concentration of a pigment, the ink composition may have higher viscosity or contain a higher concentration of coarse particles. This may cause a problem in discharge stability over a long time. Therefore, in order to achieve wide color reproducibility, it is important to select a pigment capable of exhibiting the same level of color reproducibility as that of offset printing with a low concentration of the pigment.
JP 2003-34765 A discloses a set of color inkjet inks for color printing, which not only includes magenta, yellow, and cyan inks commonly used, but also includes an ink selected from orange, green, and violet inks containing a water-soluble dye. Accordingly, a method has been proposed which provides an ink set capable of producing an image having better color gamut than an image obtained by using an ink set in the related art. However, since an increase in the number of ink colors means an increase in the number of recording heads, this method can cause an increase in the cost and size of equipment. Therefore, it is practically difficult to unlimitedly increase the number of ink colors, and thus there has been a limit to the available image quality.
In JP 2006-145703 A, a method is proposed which provides a yellow toner that uses C. I. Pigment Yellow 185 as a pigment having wide color reproducibility. However, the pigment is too coarse in this dispersion state. Accordingly, when the pigment is used as an inkjet ink, the inkjet head is clogged with the pigment particles and thus it is difficult to stably discharge the ink.
In Japanese Patent Application Nos. 2009-027361, 2010-172141, and 2010-152355 (that is, JP 2010-180376 A, JP 2011-084727A, and JP 2012-012539 A), an active energy ray-curable inkjet ink using an isoindoline-based pigment, specifically, using C. I. Pigment Yellow 185 is proposed. The C. I. Pigment Yellow 185 has very wide color reproducibility and can realize a wide range of color reproduction using a small number of ink colors. However, since a large amount of pigment precursors and by-products thereof is contained in a manufacturing process of the pigment, the pigment precursors and the by-products thereof inhibit pigment dispersion. In particular, it is very difficult to disperse the above-described pigment in a monomer. Accordingly, there is a problem in discharge stability which is the most important property as an inkjet ink, and thus it is difficult to maintain stable ejection characteristics over a long time. Moreover, a certain amount of the pigment precursors and the by-products thereof is dissolved in a monomer, and dissolution and precipitation thereof are repeated in a flow path or in a head of inkjet, which causes the ink not to be discharged. This fact causes the ejection characteristics over a long time to be further deteriorated. Accordingly, when printing is performed for a long period of time, the ink is not discharged or is deflected. Therefore, there is a problem in reliability of inkjet ink that quality of prints is degraded. As a result, although attention has been paid to the color reproducibility of the pigment, the pigment has not been widely used so far. In Japanese Patent Application Nos. 2009-027361, 2010-172141, and 2010-152355, there are also problems in that a case where ink is not discharged occurs when performing discharge for a long period of time and ejection characteristics over time are deteriorated. In addition, since the discharge stability is deteriorated at high frequency and at high ejection speed, productivity has to be lowered when used as a 1 pass-curable ink.